The most important applications are likely data processing -- batch and real-time -- and very high performance web applications. These are environments where:

The most important applications are likely data processing -- batch and real-time -- and very high performance web applications. These are environments where: * Different applications are typically isolated from one another by being on different machines. * Application and hardware failures are handled with the "let if fail" philosophy, where individual machines are treated as disposable. * Components are written in-house and typically are quite trusted (even if they don't deserve to be). People in "userspace" do, on occasion, care a lot about the overhead of syscalls. Userspace networking -- https://lwn.net/Articles/713918/ https://lwn.net/Articles/713918/ -- is a different approach, where a functionality is moved wholly out of the kernel.